Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, three-phase power at high voltage (345,000 volts phase-to-phase) is delivered to multiple transmission substations at which transformers step this high voltage down to a lower three-phase voltage (69,000 volts phase-to-phase). This 69,000-volt three-phase power then feeds multiple distribution substations whose transformers further step down the voltage to the distribution voltage (12,470 volts phase-to-phase) and separate the power into three single-phase feeder cables. Typically, these feeder cables operate at 7,200 volts phase-to-ground. Each of these feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120/240 volts for delivery to commercial and residential customers.
In many cases, the final 7,200-volt distribution network utilizes underground (i.e., buried) cables. These cables are typically known as underground residential distribution (URD) cables. Typical URD cables are shown in FIGS. 1 and 2.
In a typical URD cable 20, a central conductor 22 is surrounded by an inner semiconductor sheath 24. Inner semiconductor sheath 24 serves to relieve electrical stress by spreading out and making the electrical field more uniform.
Inner semiconductor sheath 24 is surrounded by an insulator 26. Insulator 26 has significant high-voltage insulating properties to minimize the overall size of URD cable 20. Typically, insulator 26 is formed of a polymeric material, such as polyethylene.
Surrounding insulator 26 is an outer semiconductor sheath 28. Like inner sheath 24, outer sheath 28 serves to relieve electrical stress by making the electrical field more uniform. Making the electrical field more uniform protects insulator 26, which would otherwise be more likely to break down.
Outer semiconductor sheath 28 is surrounded by a shield formed of a plurality of neutral conductors 30. Neutral conductors 30 together serve as a return line for central conductor 22. In a typical three-phase system, neutral conductors 30 carry current resulting from any imbalance among the three phases. In a mechanical sense, neutral conductors 30 form a barrier to protect URD cable 20 from casual penetration (as with a blunt shovel). In the event of a catastrophic penetration through neutral conductors 30 and into or through central conductor 22, neutral conductors 30 serve to provide a short electrical path and thereby offer some protection to a worker wielding the penetrating object.
Semiconductor layers 24 and 28 prevent high stress electrical field lines from forming under each neutral conductor 30. But as a side effect, semiconductor layers 24 and 28 also impede detection of the electrical field from outside of layer 28.
URD cable 20 may be an unjacketed URD cable 20′ (FIG. 1). In unjacketed URD cable 20′, neutral conductors 30 form the outermost layer of the cable. Neutral conductors 30 are therefore in contact with the Earth when unjacketed URD cable 20′ is buried.
URD cable 20 may also be a jacketed URD cable 20″. In jacketed URD cable 20″, neutral conductors 30 are surrounded by and embedded within an insulating jacket 32. Whether URD cable 20 is jacketed or unjacketed, neutral conductors 30 need not be grounded, but usually are grounded at the ends.
As new customers are added, URD cable 20 is cut and an extension cable is spliced in to supply power to the new customer's transformer. This poses certain problems.
One problem is that there are often multiple URD cables 20 in a given trench, conduit, or raceway. Typically, one of these URD cables 20 is de-activated prior to splicing. A problem exists in determining which of these multiple URD cables 20 is de-energized (i.e., “dead”).
Clamp-on ammeters are occasionally used in an attempt to determine if a URD cable 20 is dead. Since each URD cable 20 carries its own return, the ammeter is used to measure differential current. But a reading of zero current may result from two very different conditions. Either the cable is in-fact a dead cable, or the cable is live but nearly perfectly balanced. Since one of the goals of electrical distribution is to achieve perfect balance, the value of the test becomes more meaningless as this goal is more closely achieved. Consequently, many live cables are misdiagnosed as being dead.
Another related problem is that, in a given dig, extraneous unmapped URD cables 20 may be present. These extraneous URD cables 20 may or may not be energized, and will often confuse ammeter measurements to the point where it is impossible to determine which of the URD cables 20 is the de-energized URD cable 20 to be cut and spliced.
When a URD cable 20 is to be cut and spliced, it is first spiked. That is, a “spike” is driven through the selected URD cable 20 to short neutral conductors 30 to center conductor 22. If the spiked URD cable 20 is live, then spiking will create a short circuit and trip the appropriate circuit breakers. This assures that the worker will not cut into a live URD cable 20.
The spiking of a live URD cable 20 is undesirable for several reasons. First, spiking a live URD cable 20 poses a risk to the worker, albeit a risk significantly less than the cutting of a live URD cable 20. Second, tripping the circuit breaker causes an unnecessary power outage to all customers served by that URD cable 20. Third, unnecessarily spiking a URD cable 20 necessitates a repair of that URD cable 20. Spiking a live URD cable 20, therefore, is dangerous, costly, and time consuming.
Various apparatus have been developed to identify the status, live or dead, of URD cables 20. All of these apparatuses suffer from one or more deficiencies. When attempting to use such apparatuses to identify the status of a given URD cable 20, there are four primary conditions:    True-dead—identifying a given URD cable 20 as dead when it is in fact dead;    False-dead—identifying a given URD cable 20 as dead when it is in fact live;    True-live—identifying a given URD cable 20 as live when it is in fact live; and    False-live—identifying a given URD cable 20 as live when it is in fact dead.
A false-live result may cause the worker to backtrack and double-check the removal of power from the desired URD cable 20, may cause additional and unnecessary excavation, and may cause further labor and paperwork. This may result in a waste of time and resources. But a false-dead result, on the other hand, may lead to misidentification of the specific URD cable 20 to be cut and spliced. This is the worst possible scenario, in that the worker would then spike a live URD cable 20, believing it to be dead. As previously mentioned, spiking a live URD cable 20 is dangerous, costly, and time-consuming.
The only good status results are then a true-live and a true-false. Only such results will properly identify the specific URD cable 20 to be spiked, cut, and spliced, thereby safely, inexpensively, and efficiently allowing the work to proceed.
Apparatuses intended to determine status almost invariably test to determine if a URD cable 20 is live. No active test is performed to determine if URD cable 20 is dead. The presumption is, of course, that if URD cable 20 is not live, it is dead. This is a dangerous presumption.
If such an apparatus determines a URD cable 20 is live, it is often correct. That is, such an apparatus produces a reasonably reliable true-live result, with few false-live results. On the other hand, such an apparatus does not positively determine if URD cable 20 is dead. The apparatus can therefore only determine if URD cable 20 is “not-live”. URD cable 20 may test not-live if it is dead, or if it is live and the test fails for whatever reason, including worker error. This form of test therefore exhibits a high incidence of false-dead results. This is the worst possible scenario, in that the worker would then spike a live URD cable 20, believing it to be dead.
Another problem with many apparatuses for determining the status of URD cables 20 is that they are cumbersome to use. Often, an apparatus (or a portion of the apparatus) must be clamped to the URD cable 20 under test. This requires the worker to get down into a trench or otherwise obtain direct access to and manipulate URD cable 20.
Many such apparatuses are usable only with unjacketed URD cables 20′. Unjacketed URD cables 20 suffer from corrosion and other factors that shorten their useful lifetimes. For this reason, the cable of choice for newer installations is almost invariably jacketed URD cable 20″. In order to use an apparatus designed for unjacketed URD cable 20′ with a jacketed URD cable 20″, a portion of the insulating jacket 32 must be cut away, drilled, or otherwise penetrated. This, too, requires that the worker obtains direct access to and manipulates URD cable 20.
Because the URD cables 20 may carry high voltage (typically 7,200 volts), any procedure requiring direct manipulation of the cable is inherently dangerous. A faulty or misidentified cable may expose the worker to high voltage, and potentially precipitate injury or death. Additionally, all procedures requiring direct manipulation of the cable are cumbersome, costly, and time-consuming. This is especially true for a jacketed URD cable 20″ being tested with an apparatus intended for an unjacketed URD cable 20″. When a portion of the insulating jacket 32 has been cut away and that URD cable 20 is determined to not be the URD cable 20 to be cut and spliced, then that URD cable 20 must then be repaired to protect it from corrosion and other factors that would otherwise shorten its useful lifetime. This repair is itself cumbersome, costly, and time-consuming.
Cumbersome and time-consuming procedures often inspire workers to shortcut the testing procedure. This may lead to injury or death, as well as expensive and time-consuming error.
Even those apparatuses which do not require the worker to enter the trench often require the worker to insert the apparatus into the trench to make a measurement, then remove the apparatus from the trench to obtain the results. Such apparatuses are often difficult to maneuver from a distance. For example, one such apparatus has two probes more than 3 cm apart, and is configured to be attached to a hotstick. In order to establish a proper connection, the apparatus must be positioned so that the two probes are oriented longitudinally with the URD cable 20 and the hotstick and apparatus are perpendicular to the URD cable. This requires careful manipulation on the part of the worker. Such careful manipulation is awkward and cumbersome to perform in the field, and inspires the worker to shortcut the testing procedure by making a minimum number of tests (often a single test, especially where a desired result is obtained) where a plurality of tests is required for positive results.
Many apparatuses for determining the status of a URD cable 20 do so by detecting the presence of an electric field in or around a live URD cable 20. This field is very weak, on the order of a few millivolts at best. Since the URD cables carry high voltages at respectable currents, the environment in which the tests are performed is generally electrically noisy. The combination of low signal strength and a noisy environment creates a tendency for such apparatuses to indicate false-live statuses. This is especially true of those apparatuses having lengthy and or unshielded input lines. Of course, an apparatus may be designed to obtain fewer false-live readings at the expense of more false-dead readings. But this would only lead to a less usable apparatus. Since the false-dead condition is even worse than the false-live condition.
Many apparatuses have a first portion contacting the URD cable 20 under test and a second portion indicating the test results, where the first portion is naturally in the trench or raceway with the URD cable 20 and the second portion is with the operator. Alternatively, many apparatuses derive power from a generator or work vehicle. In both cases, there is a cable or line extending from the URD cable to a person or object outside of the trench or raceway. This poses a significant hazard in that a defective URD cable 20, or a penetration of an otherwise good URD cable 20, may cause the high voltage to be conducted over the line to a point outside the trench or raceway. Once high voltage is out of the trench or raceway, there is a danger that an individual may come into contact with the high voltage and suffer injury or death as a result. In addition, this high voltage may find a path through equipment which may subsequently become damaged or destroyed.